The present invention relates to an oriented polyethylene film and a process for producing the same, particularly to a biaxially oriented polyethylene film excellent in tearability and a process for producing the same by biaxially stretching a polyethylene sheet crosslinked in a specific manner, under specific conditions.
In the fields of food packaging and drug packaging, packages such as standing pouches and portioned packs are required to be longitudinally linearly tearable at their cut lines. For such applications, longitudinally uniaxially oriented polyethylene films have previously been used. These films are excellent in linear tearability in their stretched direction. However, these films have the disadvantage of providing unclear cut lines and hence being inferior in clear tearability, because the films are generally stretched at high draw ratios, which causes the films to be fibrillated, and therefore fibrils are liable to be produced on tearing.
On the other hand, rolled oriented polyethylene films are commercially available for these applications. These films are characterized by depressed occurrence of fibrils and good tearability in a longitudinal direction, but have the disadvantage that the longitudinal strength is not balanced with the lateral strength.
Furthermore, in order to improve the inferior clarity of prior-art films formed of polyethylene produced by a medium- or high-pressure process, there has been proposed a process comprising crosslinking homogeneously a film formed of an ethylenic resin such as polyethylene by radiation and then stretching biaxially the crosslinked film (Japanese Patent Pablication No. 18893/1962). This film is improved in clarity, but insufficient in moistureproofness. Hence, for purposes in which better moistureproofness is required, the improvement in moistureproofness has been desired.
Then, the present inventors have previously proposed an oriented polyethylene film in which the degree of crosslinking inwardly decreases across the thickness of the film (Japanese Patent Unexamined Publication Nos. 174322/1984, 74819/1986 and 74820/1986). This oriented film is composed of both crosslinked outer layers and an uncrosslinked intermediate layer, and can meet the excellent moistureproofness requirement described above. It is unnecessary to provide further a carrier layer or to increase the thickness of the film for the purpose of improving the moistureproofness. This oriented film has therefore various advantages that the clarity and the packaging properties of the film are not deteriorated.
The above-mentioned oriented film proposed by the present inventors is produced by stretching a rolled sheet preferably at a temperature ranging from the softening point of a raw material resin to the melting point thereof. However, the oriented film obtained under such conditions is somewhat insufficient in terms of the possession of both the linear tearability and the clear tearability described above, particularly for the application of standing pouches or the like in which it is required to have both these properties. Furthermore, the longitudinal elongation and strength (tearing strength) are insufficiently balanced with the lateral elongation and strength (tearing strength), respectively.